This invention relates in general to flow control valves for drill stem test tools of the type used to test oil producing formations and specifically to flow control valves which are opened and closed in response to external pressure in the well annulus.
Drill stem test (DST) tools are mounted in the drill stem or string and are used to evaluate the producing potential or productivity of an oil or gas bearing zone prior to completing a well. Thus, as drilling proceeds, various indications such as core samples may suggest the desirability of testing a certain formation for producing potential. To conduct the test, a packer and valve assembly is lowered on the drill stem into the uncased well bore to the zone to be tested. The packer is then set and the valve is opened for flow to the well surface.
Various techniques have been utilized to open and close DST valves once the tool has been placed in the well bore. Such techniques commonly comprise rotating the drill stem in a clockwise or counter-clockwise direction, sometimes coupled with lifting up or setting weight down on the tool from the surface. Such techniques are satisfactory in straight well bores such as are commonly encountered on land but are problematical in deviated well bores of the type commonly employed in off-shore drilling operations. A need exists, therefore, for a DST valve which is operable between open and closed positions with a minimum of mechanical manipulation of the drill stem. One solution to this problem is to incorporate an operating means in the DST valve which moves the valve between open and closed positions in response to pressure in the surrounding well annulus. The well bore can then be enclosed and "pressured-up" to operate the valve. Since annulus pressure varies with depth, unexpected variations in pressure can cause the valve to open prematurely. The tool must, therefore, be designed to compensate for variations in the hydrostatic head in the well annulus as the tool is placed and retrieved from the well bore.